Since an announcement of an InGaZnO4 (a-IGZO) thin film transistor (TFT) that exhibits the mobility of a-Si or higher in the amorphous state, researches and developments of oxide semiconductors for practical use have been energetically conducted in all over the world. Most of these oxide semiconductor materials are however n-type oxide semiconductors using electrons as carrier.
If a p-type oxide semiconductor that is comparative to the n-type oxide semiconductor can be utilized on practice, these p-type oxide semiconductor and n-type oxide semiconductor are combined to form p-n junction, to thereby realize a diode, an optical sensor, a solar battery, a LED, or a bipolar transistor. It is also possible to make these devices transparent, as the oxide semiconductor can be designed to have a wide-band gap.
Moreover, an active-matrix display uses, as a basic driving circuit, a 2T1C circuit, as illustrated in FIG. 7. In such display, if a driving transistor (field-effect transistor 20) is n-type, the driving circuit is so called source follower circuit. Therefore, the operating point of the driving transistor is sifted to the operating point of the different gate voltage due to aging (especially increasing in voltage) of the properties of the organic EL element, to thereby shorten half-life of the display. For this reason, it has not yet realized an active-matrix organic EL display (AM-OLED) using a-IGZO TFT of high mobility as a backplane, p-type low temperature polysilicon thin film transistors (LTPS-TFT) have been exclusively employed. Therefore, there is a need for a p-type oxide semiconductor of high performance.
It has been known since 1950s that a Cu2O crystal that is a monovalent copper oxide exhibits p-type conductivity (see, for example NPL 1). This crystal has a dumbbell structure of O—Cu—O as a base, and the hybrid orbital of the 3d-orbital of Cu and 2p-orbital of oxygen constitutes a top of a valence band. Holes are introduced into the valence band by oxygen excess stoicheimetry, to thereby exhibit p-type conductivity.
Examples of the crystal having the dumbbell structure as a base unit include a delafossite crystal represented by CuMO2 (M=Al, Ga, In), and SrCu2O2 crystal. In order for these oxides to exhibit p-type electric conductivity, they need to have high crystallinity, and only crystals, p-type conductivity of which has been reported, are CuAlO2, CuInO2, and SrCu2O2 (see, for example, NPL 2 to NPL 4).
One of the reasons why it is difficult to exhibits p-type conductivity is that the valency of Cu and oxygen content are difficult to control. When it is attempt to form a single phase film of oxide containing highly crystalline Cu+, a resulting film is, in most cases, a film in which a crystal phase containing Cu2+, such as CuO, SrCuO2, and SrCu2O3 is mixed. With such film, excellent p-type electric conductivity cannot be attained, and it is difficult to control properties thereof. Accordingly, when these p-type oxide materials are used as an active layer of a semiconductor element, properties, such as carrier density and carrier mobility, cannot be optimized.
Other than above, disclosed is delafossite oxide containing monovalent Cu or Ag (for example, see PTL 1). However, in the disclosed technique, a heat treatment of high temperature, 500° C. or higher, is necessary and therefore it is not suitable for practical use.
Moreover, disclosed is a p-type electric conductive thin film of crystalline SrCu2O2 (for example, see PTL 2). In the disclosed technique, the thin film forming temperature is relatively low, i.e., 300° C., but sufficient electric conductivity has not been attained, and the best conductivity attained is 4.8×10−2 Scm−1. Further, control thereof is also insufficient.
As described above, these disclosed techniques have a problem that a p-type oxide material cannot be produced by a practical method, and a p-type oxide that has appropriately controlled and sufficient electric conductivity cannot be provided.
Moreover, disclosed is a TFT using, as an active layer, a p-type oxide material delafossite crystal containing monovalent Cu or Ag for example, see PTL 3).
However, this literature does not disclose sufficient information, such as properties of a material of the active layer, a formation method thereof, and properties of the transistor.
Further, disclosed is a TFT using a Cu2O crystal as an active layer (see, for example, NPL 5 and NPL 6). In accordance these techniques, however, properties of the active layer cannot be sufficiently controlled, and therefore the electron field-effect mobility and on-off ratio of the TFT have not reached the levels suitable for practical use.
As described above, these disclosed techniques have problems that it is difficult to control properties of the p-type oxide material, such as carrier density, and the properties suitable for use in a device cannot be attained.
So far, it has not yet been discovered a p-type oxide that is effective in practical use.
Accordingly, there is currently a need for a p-type oxide having characteristics that are comparative to characteristics of an n-type oxide.